Diff reduce with upstream and consolidate #ifdefs where possible

Clarify why sys/types.h #include is needed on FreeBSD

Port contrib/netbsd-tests/kernel/kqueue/... as tests/sys/kqueue/...

proc2_test must be skipped because the invariant tested
(`ke.fflags & NOTE_TRACKERR`) doesn't pass.

MFC after: 58 days
Submitted by: kib (earlier form)
Sponsored by: EMC / Isilon Storage Division
Differential Revision: https://reviews.freebsd.org/D6172